This invention relates to an antilock control method for vehicle wheel brakes.
When the brakes of a vehicle are applied, a braking force is generated between the wheel and the road surface that is dependent upon various parameters which include the road surface condition and the amount of slip between the wheel and the road surface. This braking force increases as slip increases until a critical value of slip is surpassed. Beyond the critical value of slip, the braking force decreases and the wheel rapidly approaches lockup. Therefore, to achieve stable braking, an antilock control system seeks to operate wheel slip at or near the critical slip value. An antilock control system achieves this objective by detecting an incipient wheel lock condition. Upon detecting an incipient wheel lock condition, the antilock control system releases pressure at the wheel brake to allow recovery from the incipient wheel lock condition. Upon recovery, brake pressure is reapplied. Criteria used to indicate an incipient wheel lock condition includes excessive wheel deceleration and/or excessive wheel slip.
One known antilock control system uses a motor driven pressure modulator in which a DC torque motor drives a piston in a cylinder whose volume is modulated to control the hydraulic brake pressure at the wheel brake. In this system, because of the relationship between motor current, motor torque and motor load represented by the hydraulic brake pressure on the head of the piston, the value of motor current is used as a representation of brake pressure and is controlled to provide control of the brake pressure. In one such system, when an incipient wheel lock condition is sensed, the value of motor current at this time is stored as a representation of the brake pressure producing the maximum braking force coexisting with the critical slip between the wheel and the road surface and the motor current is controlled to quickly retract the piston to release brake pressure to allow recovery from the incipient wheel lock condition. When a recovery from the incipient wheel lock condition is sensed, the motor current is controlled to extend the piston to re-apply brake pressure. In re-applying the brake pressure, the pressure is quickly established substantially at the brake pressure producing the maximum braking force by quickly establishing the motor current at a significant fraction of the motor current stored at the time an incipient wheel lock condition was sensed. Thereafter, brake pressure is ramped at a controlled rate which may be a function of wheel slip and acceleration by ramping the motor current in direction applying brake pressure until an incipient wheel lock condition is again sensed after which the cycle is repeated.
Typically, in these systems the brake pressure is ramped by repeatedly increasing the motor current in stepwise fashion. In this form of control, the motor current is incremented by some determined step value at repeated time intervals. The ramp rate of the current and therefore the rate of increase of the applied brake pressure is established by varying one or both of the step value and the value of the repeated time interval. When the current is increased in this stepwise fashion, there is a potential for the motor to stop when the brake pressure/motor torque balance is reached prior to the next step increase in the motor current. When this condition occurs, the motor may remain stationary even after the next step increase in the motor current as a result of the static friction of the modulator which must be overcome before movement will again begin. While the increase in motor torque resulting from the current step is sufficient to overcome the dynamic friction of the modulator, if the amount of the current step is not adequate to increase the motor torque by the amount required to overcome this static friction which is greater than the dynamic friction, the motor will not rotate and the brake pressure will not be ramped. The motor will again rotate to increase pressure only when the motor current is increased by additional subsequent steps to the level such that the static friction is overcome. The effect of the foregoing is an erratic increase in the brake pressure during the ramping portion of the antilock braking cycle.